The present invention relates to current driving devices, and more particularly relates to a technique used for a preferable current driving device as a display driver such as an organic EL (electro luminescence) panel.
In recent years, the size and definition of flat panel displays such as organic EL panels have been increased. At the same time, flat panel displays have been made thinner and lighter and also costs for fabricating such panels have been reduced. In general, an active matrix method is preferably used as a driving method for a large-size, high definition display panel. Hereinafter, a known display driver for an active matrix display panel will be described.
FIG. 20 is a circuit diagram illustrating the configurations of a display panel and a known current driving device serving as a display driver connected to the display panel. Herein, a display panel means to be an organic EL panel.
As shown in FIG. 20, the known current driving device includes current supply sections 1001a1, 1001a2, . . . and 1001an (hereinafter, referred to as a “current supply section 1001a when the current supply sections are not distinguished from each other) for supplying driving currents to a plurality of pixel circuits 1005a1, 1005a2, . . . and 1005an (hereinafter, referred to as a “pixel circuit 1005a when the pixel circuits are not distinguished from each other) formed on a display panel, respectively, and a reference current supply section (bias circuit) 1101 for supplying a reference current to each section in the current supply section 1001a. Note that a “reference current” herein means to be not only a predetermined current flowing from a reference current generator but also a current from a reference current generator transmitted by a current mirror circuit.
When the size of a display panel is large as in a television display device, a large number of current supply sections 1001a are provided so that the current supply sections 1001a are divided onto a plurality of semiconductor chips 1105. The semiconductor chips 1105 are arranged in a frame portion of a display panel in many cases.
Each of the pixel circuits 1005a1, 1005a2, . . . and 1005an includes a p-channel first TFT (thin-film-transistor) 1104 connected to the current supply section 1001a via a signal line, a second TFT 1102 constituting a current mirror circuit together with the first TFT 1104, and an organic EL device 1103 for emitting light in accordance with a current supplied from the second TFT 1102.
The reference current supply section 1101 includes a p-channel first MISFET 1108 to which a supply voltage is supplied at a terminal, a resistor 1107 which is connected with the first MISFET 1108 and generates a reference current, a p-channel second MISFET 1109 constituting a current mirror circuit together with the first MISFET 1108, and an n-channel current input MISFET 1110 for transmitting a reference current to the current supply portion 1001a. 
When a gray scale of m bits is controlled, each of the current supply section 1001a includes current sources 1112-1, 1112-2, . . . and 1112-m (m is a positive integer) arranged in parallel to an output section connected to the pixel circuit 1005a and switches 1115-1, 1115-2, and 1115-m for controlling currents flowing in the current sources 1112-1, 1112-2, . . . and 1112-m, respectively, to turn the current ON or OFF. Herein, each of the current sources 1112-1, 1112-2, . . . and 1112-m is formed of an n-channel MISFET 1110 constituting a current mirror circuit together with the current input MISFET 1110. Moreover, each of the switches 1115-1, 1115-2, . . . and 1115-m independently performs a switching operation based on display data.
FIG. 23 is a circuit diagram illustrating the arrangement and configuration of a current supply section in the known current driving device. In FIG. 23, an example of current supply sections for 64 gray scale in which six current sources are provided in each current supply section is shown. The current sources 1112-1, 1112-2, . . . and 1112-m include an MISFET, two MISFETs, . . . and 32 MISFETs having the same size and properties, respectively. These MISFETs are arranged in the manner shown in the upper part of FIG. 23 when viewed from the top. Connections are made so that adjacent ones of the MISFETs are connected to a single output section.
With the above-described configuration, the current supply section 1001a is substantially operated as a current mode D/A converter, receives display data as a digital signal, and withdraws as an analog signal a current having an amount corresponding to the display data from an output section.
As has been known, an organic EL device has the rectifying action as a diode does and the illuminance of the organic EL device varies in accordance with the amount of a current conducted. In the pixel circuit 1005a, the amount of a current conducted by the organic EL device 1103 varies in accordance with the amount of a current flowing in the TFT 1104. Accordingly, the organic EL device 1103 is current-driven by the current supply section 1001a, so that the illuminance of the organic EL device 1103 varies.
As has been described, the current driving device current-drives the plurality of pixel circuits 1005a in the display panel based on display data to achieve a gray scale display (see, e.g., Japanese Unexamined Patent Publication No. 11-88072 and Japanese Unexamined Patent Publication No. 11-340765).